It turns out a lot of what we throw in the garbage is stuff that could be composted. Stuff like food scraps, used Kleenex and greasy pizza cartons.

In Duluth, the Sanitary District runs a compost site that turns 50-foot-long piles of anything you can think of into compost.

The district's Susan Darley-Hill calls these piles windrows, because they look like giant rows of hay, only brown. It takes a couple of days to put together enough material to make these windrows, and each one has a small electric pump at the end to force air through it.

"When the windrow is complete, we turn that air on, and that will circulate through the pile," Darley-Hill explained. "That keeps the 'bugs,' as we call them, the microorganisms, well supplied with oxygen, which is a really critical part of digestion process."

A thermometer with a long stem reaches deep into the pile.

"It looks like it's reading about 143 degrees Fahrenheit, which is a great temperature," said Darley-Hill. "It's probably about 10 degrees outside right now but it's really toasty warm inside."

Most of the material for this gardener's black gold comes from hospitals, colleges and restaurants -- places that produce a lot of food waste. It's delivered daily, and mixed with the leaves and grass clippings that people bring from their yards.

The manager carefully calculates the nitrogen and carbon content of all the materials, to create a perfect recipe that will cook just right.

Duluth and the other towns served by the sanitary district may be the only places in the U.S. with a law that requires big producers of food waste to separate it from their trash and deliver it here.

Most people don't realize how important recycling is, says Ginny Black of the Minnesota Pollution Control Agency. Her job is to get more people and businesses to compost their waste.

"Aside from giving up your car, and walking and riding your bike everywhere, the biggest impact you can have in reducing the greenhouse gases is to recycle," said Black.

There's a direct connection between the amount of garbage we send to landfills, and greenhouse gases that lead to global warming.

First of all, when organic material sits in a landfill, it decomposes without oxygen. That produces methane -- a greenhouse gas that's 20 times more potent than the carbon dioxide that comes from smokestacks or your car's tailpipe.

Second, it takes less energy to recycle than to create new raw materials, and that includes soil.

Third, using compost returns carbon to the soil, keeping it out of the atmosphere, where it contributes to global warming.

"Most of what's left after composting is a carbon substance, the organic material," Black said. "So when you incorporate that back into the soil, it's like a bank, it's like a savings account. In this case it's carbon, and we're putting the carbon back in the soil."

The students at Susan Lindgren Elementary School are helping make compost. They're happy to explain the system to a visitor.

"You see there's a plastic and trash bin, and there's a food and paper bin," said one student.

They dump the trash in one bin, put their uneaten food in the other, and stack the plastic cups and paper trays on a table. The cups will be recycled and the paper trays will join the food waste as it heads to the compost pile.

That reduces the amount of stuff going to the incinerator.

"Because some people burn it, and it pollutes the air, and we're trying to stay healthy and free of that," another student explained.

One less weekly haul to the incinerator helps offset the pickup cost for the compost materials. And the state doesn't charge its 17 percent waste tax on the compostable material.

Principal Ann Sullivan is the driving force behind this move at Susan Lindgren Elementary School. When kids learn about the benefits of composting, they teach their families, and that could be one way of moving Minnesota back to the forefront of the recycling movement, she said.

Right now the program depends on grants to keep costs down, but she thinks it will pay for itself, eventually.

"Just like years ago no one could imagine separating out your glass, your cans, and your paper, everybody does that, that's just how we do business now," she says. "And this is the next wave of that."

Some restaurants in the Twin Cities participate in a voluntary food composting program.

A few cities already have curbside pickup of compostable materials. Both St. Paul and Minneapolis are trying to figure out how to make it work.